The power dissipation of any series regulator is equal to the output current times the input-to-output voltage differential which, for monolithic regulators, is on the order of 2-3 volts. For low-voltage supplies this differential results in high inefficiency. When coupled with switching power supplies as a post-regulation stage, the power losses due to the series regulator become disproportionally large and adversely affect the high efficiency one would normally now expect from a switching power supply.
Since the output load current of a regulator is generally fixed by its application requirements, it becomes obvious that to reduce power dissipation and improve efficiency, the input-to-output voltage differential must be minimized. And since present day practice results in a 2 to 3 volt input-to-output differential, a highly undesirable state of affairs exists in the three-terminal regulator art.
In an attempt to improve this situation, Unitrode Corporation has recently introduced a high efficiency series IC regulator which utilizes an external PNP bipolar transister as a series pass element (see Electronic Design News (EDN) May 17, 1984, pp. 161-175). Although this configuration is an improvement in reducing the input-to-output voltage differential, considerable base drive power is dissipated due to the bipolar transistor's variable Beta over temperature. Also, any additional load current requires a Darlington configuration which increases the input-to-output voltage differential.
U.S. Pat. No. 3,983,473 issued to Sanderson and U.S. Pat. No. 4,005,353 issued to Yokoyama, both utilize Depletion Mode FET devices to maximize efficiency in a series regulator. However, these designs are restricted to low power circuits since Depletion Mode FETs are low powered devices.